Engine is still overheating

[20pilot]

I have a inboard mercruiser 5.7 with a throttle body. I have been having an overheating issue so I started with all of the main parts:

Raw Water Pump, replaced impeller
New Circulating pump
Heat exchanger boiled and pressure tested
New Manifolds and riser
Inspected all hoses, no restrictions
Removed thermostat to check and still overheating

So I just ran the boat, 3000 rpm ran good at 150 for 15 min, then temp started to creep up to 175. Came off of plane, put the boat in neutral, rev'd engine to 2k rpm, and temp started to go down. Within 3-5 min it was back at 145/150. Brought it back to idle, then shifted into gear, ran at 2900, again started to creep back up. I just hauled the boat and the strainer is clean under the boat. I am at a loss for what it could be... Oil Cooler?Any Ideas or suggestions?

 

[alldodge]

Could use a serial number or at least a year to see what should be there. With a heat exchanger I would say you have a belt driven raw water pump.
What does the hull look like, and how about the thru hull pick up?
Are you using T-bolts for hose clamps or worm clamps?
Does your thermostat housing have check balls?

 

[Bt Doctur]

for a fresh water cooled motor 170-175 is not hot, have you checked the base timing and the advanced timing

 

[alldodge]

for a fresh water cooled motor 170-175 is not hot, have you checked the base timing and the advanced timing

Dang :facepalm: , great point BtDoc, it blew right past me. This is a closed cooled motor, might have a 180 degree thermostat

 

[20pilot]

This is a new boat to me that I purchased used. Its a 23' Fish Nautique, with a 5.7 but I am not able to locate and serial numbers. I did pull the thermostat and run it with out one and still the temperature went to 175/180. I thought without a thermostat it would stay around 150-155.

 

[alldodge]

What year is it?

 

[Bt Doctur]

without a thermostat is should be 100 but you can still overheat without one. you say "inboard" so you have a engine mounted raw water pump?
I would start with a 160 Deg stat , make sure the impeller is in good condition. That all the hose clamps are tight so your not sucking air.
We don't know ( the seacock and scoop, or screen)where, what type, or how its installed , could be turbulence bubble getting sucked in

 

[thejeep007]

Last year my boat overheated when I I was up on plane only, took me forever to track down the problem. The hose from the lower unit to gimble housing was dry rotted and would suck air on a plane sometimes. Hope this helps!

 

[Bt Doctur]

you say you replaced the impeller but did you replace the water pump base? If it is a air bubble issue the only place you can suck air is from the bottom of the water pump base. This is above the water line at planning speed and can suck in the hot exaust.Can be checked by dropping the lower unit and putting muffs on it with full water pressure.Any leak would be evident and its where you can suck air in.

 

[20pilot]

No clue on the year as I bought it used. The cover that normally has the serial/model is not there. This boat is a straight inboard with stuffing box. I replaced the water pump with new wear plate, impeller and housing. The bottom has a thru hull with strainer about a foot or so off the keel aft of the engine. I ran the boat today, at about 10 min of run time with no thermostat temp got up to 150, I turned back and by the time I came off plane it was 170.

 

[20pilot]

I looked again yesterday, no model or serial on the engine that I could see.

 

[Bondo]

I looked again yesterday, no model or serial on the engine that I could see.

Ayuh,..... Look for a tag riveted to the block just above the starter, or on the block flange above the starter,.....

What year is the boat,..??

 

[alldodge]

I looked again yesterday, no model or serial on the engine that I could see.

There are so many differences in cooling systems I'm shooting in the dark. Here is a link to a 1988 to 1993, see if the system looks like your
http://www.mercruiserparts.com/Show_...Cooling+System